The IEEE 1394b serial bus protocol provides protocols and arbitration mechanisms for transmission speeds up to “S3200,” which translates to 4 GBaud over a connecting cable due to 8B10B encoding.
However, the IEEE 1394b standard does not provide the details for the electrical signaling, due to lack of sufficient experience/evidence etc at the time the standard was prepared.
The electrical specifications for the IEEE 1394 family of protocols need to use some form of “de-emphasis” to overcome issues of intersymbol interference (ISI). ISI describes the degradation of signal quality on a transmission line due to frequency-dependent attenuation within the transmission line—typically higher frequencies are attenuated more than lower frequencies. This phenomenon is similar to an instance where bass notes from a loud in-car entertainment system can be heard as the car passes by, but no other music accompanying the bass can be heard. Typically in digital transmission a sequence of bits of the same value generates, in effect, a lower frequency signal that affects the following bit if this is a single bit, which generates a higher frequency signal. The results are typically measured at the receiver by means of an eye diagram that overlays a large number of received signals, aligned such that the center of the bit time for each signal is coincident. If there is no signal degradation, at the center time all the signals will be high or low (depending on whether a “1” bit or a “0” bit is being transmitted), resulting in a wide-open “eye.” If signal degradation occurs, then at the center of the bit time some of the signals will not be so high or low and the eye will be less open. If the eye is fully closed, then it is impossible to determine whether the transmitted bits are 1 or 0 without at least some contextual information.
De-emphasis is a technique in which a PHY's transmitter modifies the amplitude of the signal for various runlengths of multiple identical bits so that the signal as received at the receiver is free of apparent ISI. Essentially, the transmitter pre-compensates for the ISI that is introduced as the signal traverses the cable. There is an alternative technique of equalization in the receiver that applies to DSP techniques for extracting contextual information, but for various technical reasons, the technique is less favored.
The amount of de-emphasis to apply in any particular case is likely to depend on the attenuation properties of the particular cable being used. With very short cables, there may be little or no de-emphasis required, whereas with long cables a considerable amount of de-emphasis may be appropriate. The consequence is that it is necessary to perform some form of training of the transmitter when a connection is established. Such training enables the transmitter to determine how much de-emphasis to apply for the actual cable being used in the connection. The use of de-emphasis determined as the result of analysis of the particular connection is generally known as adaptive de-emphasis.
At relatively low transmission rates, de-emphasis is not required. At somewhat faster transmission rates, fixed de-emphasis is adequate. But for higher performance, a connection requires some form of adaptive de-emphasis. Furthermore, a connection is clearly much more robust with some form of adaptive de-emphasis. However, there are no effective adaptive de-emphasis methods because training has proven to be difficult and complex, and this was one reason why there was no specification included in 1394b for signaling at the S3200 rate. A prior approach has been to utilize the existing connection negotiation signaling in 1394b. In such an approach, a receiving node analyzes the signal (referred to as connection tones) to determine the amount of desired de-emphasis. However, this approach suffers significant shortcomings. Each tone comprises a relatively low frequency signal. Hence the bit patterns sent during existing connection negotiation signaling are not suitable for analysis to determine the amount of high frequency attenuation inherent in the transmission line (and hence the amount of de-emphasis to be applied), as they are of too low frequency, and even if such analysis could be performed by the receiver, there is no way to communicate the result of any such analysis to the transmitter. Thus, there is a heartfelt need for a de-emphasis training mechanism that works effectively.